Generally, a conventional automatic transmission used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism.
A hydraulic control system for the automatic transmission operates by the selective supply of hydraulic pressure, the flow of which is generated by a hydraulic pump, to each friction element (for engagement or disengagement of the same) by a plurality of control valves such that shifting can be realized automatically and appropriate to the driving state of the vehicle.
Such hydraulic control system generally comprises a hydraulic fluid source, a line pressure controller for regulating hydraulic pressure supplied from the fluid source to line pressure, a range controller for selectively supplying hydraulic pressure from the line pressure controller, a shift controller for supplying hydraulic pressure from the range controller to lines corresponding to the respective transmission speeds by control of a transmission control unit (TCU), a hydraulic pressure controller for converting hydraulic pressure from the range controller to operational pressure operating the friction elements, and a hydraulic pressure distributor for determining a hydraulic flow path corresponding to the respective transmission speeds according to the hydraulic pressure from the shift controller and suitably distributing the operational pressure to each friction element.
Namely, automatic shifting is realized by selectively operating the friction elements through hydraulic pressure controlled by the above controllers in accordance with a driving state of the vehicles and engine throttle opening.
Referring to FIG. 1, there is shown a hydraulic circuit diagram of a conventional hydraulic control system. The hydraulic control system includes a torque converter 2 for transmitting power from an engine to the transmission, a damper clutch control valve 4 for controlling a damper clutch mounted in the torque converter 2 to increase power transmitting efficiency, a line pressure regulating valve 8 for regulating hydraulic pressure from a fluid pump 6, and a reducing valve 10 for supplying reduced hydraulic pressure to solenoid valves S1 and S4, and the damper clutch control valve 4.
A manual valve 12 receives line pressure from the fluid pump 6 such that the line pressure is supplied to the line pressure regulating valve 8 and a shift control valve 14.
The shift control valve 14 selectively supplies hydraulic pressure from the manual valve 12 to each friction element such as a front clutch 24, a rear clutch 26, a low/reverse brake 28, an end clutch 30, and a kickdown servo 32 through control of the hydraulic pressure distributor having a plurality of spool valves such as a 1-2 shift valve 16, an end clutch valve 18, 2-3/4-3 shift valve 20, and a rear clutch valve 22.
The manual valve 12 is connected to a N-D control valve 34 for reducing shift shock when port conversion of the manual valve from a neutral N range to a drive D range, through a first speed line L1. Further, the N-D control valve 34 is connected to a pressure control valve 36 which controls hydraulic pressure in the first speed line L1 and supplies the controlled pressure to the 1-2 shift valve 16 via the N-D control valve 34 when in second, third and fourth speed.
Between the manual valve 12 and the 1-2 shift valve 16 is provided a N-R control valve 38 for reducing shift shock when port conversion of the manual valve 12 from the neutral N range to a reverse R range such that the N-R control valve 38 supplies hydraulic pressure from the manual valve 12 to the low/reverse brake 28 via the 1-2 shift valve 16.
The shift control valve 14 supplies hydraulic pressure to the 1-2 shift control valve 16 through a second speed line L2 to control a valve spool of the 1-2 shift valve 16. Further, the shift control valve 14 supplies hydraulic pressure to the 2-3/4-3 shift valve 20 through a third speed line L3 to control a valve spool of the 2-3/4-3 shift valve 20. Simultaneously, the hydraulic pressure in the third line L3 is supplied to the end clutch 30 via the end clutch valve 18.
Further, the shift control valve 14 supplies hydraulic pressure to the rear clutch valve 22 through a fourth speed line L4 to control a valve spool of the same. Simultaneously, the hydraulic pressure in the fourth speed line L4 is supplied to the end clutch 30 via the end clutch valve 18.
Meanwhile, between the line pressure regulating valve 8 and the damper clutch control valve 4 is provided a torque converter control valve 39. Reference numerals S1, S2, S3, and S4 which have not yet been described are solenoid valves ON/OFF or duty controlled by a transmission control unit (TCU).
In such a hydraulic control system, line pressure is regulated by three stages; parking range "P", drive range "D", and reverse range "R". In the middle of regulating line pressure, what is most important is to maintain the line pressure to a uniform level when in the drive "D" range.
In such drive "D" range state, when fluid flowing in the automatic transmission has a high temperature or RPM of the engine is low such that amount of fluid supplied from the fluid pump 6 is insufficient, a phenomena in which line pressure is reduced occurs.
Further, when a shift process starts to proceed or when standby friction elements start to operate, hydraulic pressure is temporarily reduced such that hydraulic pressure is not uniform. Accordingly, shift quality is reduced, too.